Lithographic apparatus and a device manufacturing method

ABSTRACT

A substrate table for an immersion system having a projection system arranged to project an image onto a substrate and a liquid confinement system configured to confine an immersion liquid to a space between the projection system and the substrate, the substrate table including: a substrate holder configured to hold a substrate; and a current control device arranged to reduce an electric current flowing between the substrate and the substrate holder while the immersion liquid is confined to the space.

This application claims the benefit of priority of European PatentApplication No. 18210662.5, filed Dec. 6, 2018, which is incorporatedherein in its entirety by reference.

FIELD

The present description relates to a lithographic apparatus, inparticular of the immersion type and a device manufacturing method usingthe lithographic apparatus.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may, for example, project a pattern (also often referred to as“design layout” or “design”) of a patterning device (e.g., a mask) ontoa layer of radiation-sensitive material (resist) provided on a substrate(e.g., a wafer).

As semiconductor manufacturing processes continue to advance, thedimensions of circuit elements have continually been reduced while theamount of functional elements, such as transistors, per device has beensteadily increasing over decades, following a trend commonly referred toas “Moore's law”. To keep up with Moore's law the semiconductor industryis chasing technologies that enable to create increasingly smallerfeatures. To project a pattern on a substrate a lithographic apparatusmay use electromagnetic radiation. The wavelength of this radiationdetermines the minimum size of features which are patterned on thesubstrate. Typical wavelengths currently in use are 365 nm (i-line), 248nm, 193 nm and 13.5 nm.

In an immersion lithographic apparatus an immersion liquid is interposedin a space between a projection system of the apparatus and a substrate.In this specification, reference will be made in the description tolocalized immersion in which the immersion liquid is confined, in use,to the space between the projection system and a surface facing theprojection system. The facing surface is a surface of substrate or asurface of the supporting stage (or substrate table) that is typicallyco-planar with the substrate surface. (Please note that any reference inthe following text to “surface of the substrate” also refers in additionor in the alternative to a surface of the substrate table, unlessexpressly stated otherwise; and vice versa.) A fluid handling structurepresent between the projection system and the stage is used to confinethe immersion liquid to the immersion space. The space filled by liquidis typically smaller in plan than the top surface of the substrate andthe space remains substantially stationary relative to the projectionsystem while the substrate and substrate stage move underneath.

SUMMARY

In a lithographic apparatus the substrate to be exposed (which may bereferred to as a production substrate) is held on a substrate holder(sometimes referred to as a wafer table). The substrate holder may besupported on a substrate table which is moveable with respect to theprojection system. The substrate holder usually comprises a solid bodymade of a rigid material and having similar dimensions in plan to theproduction substrate to be supported. The substrate-facing surface ofthe solid body is provided with a plurality of projections (referred toas burls). The distal surfaces of the burls conform to an essentiallyflat plane and support the substrate. The burls provide severaladvantages: a contaminant particle on the substrate holder is likely tofall between burls and therefore does not cause a deformation of thesubstrate; it is easier to machine the burls so their ends conform to aplane than to make the surface of the solid body flat; and theproperties of the burls can be adjusted, e.g. to control the clamping ofthe substrate.

However, the burls of the substrate holder wear during use, e.g. due tothe repeated loading and unloading of substrates. Uneven wear of theburls leads to unflatness of the substrate during exposure which canlead to a reduction of the process window and, in extreme cases, to animaging error. Due to the very precise manufacturing specifications,substrate holders are expensive to manufacture so that it is desirableto increase the working life of a substrate holder.

It is desirable, for example, to provide improved means to reduce wearof substrate holders, especially in an immersion type lithographicapparatus.

According to an aspect, there is provided a lithographic apparatuscomprising:

a substrate holder configured to hold a substrate;

a projection system arranged to project an image onto a substrate heldby the substrate holder;

a liquid confinement system configured to confine an immersion liquid toa space between the projection system and the substrate; and

a current control device arranged to reduce an electric current flowingbetween the substrate and the substrate holder while the immersionliquid is confined to the space.

According to an aspect, there is provided a device manufacturing methodcomprising using a lithographic apparatus having a projection system toproject an image onto a substrate held on a substrate holder, the methodcomprising:

using a liquid confinement to confine a liquid to a space between theprojection system and the substrate; and

reducing a current between the liquid confinement system and thesubstrate holder while the immersion liquid is confined to the space.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 schematically depicts a lithographic apparatus;

FIG. 2 schematically depicts an immersion liquid confinement structurefor use in a lithographic projection apparatus;

FIG. 3 is a side cross-sectional view that schematically depicts afurther immersion confinement structure according to an embodiment;

FIG. 4 is a map showing variation in burl height of a used substrateholder;

FIG. 5 is a map showing humidity in the space between a substrate holderand the substrate;

FIG. 6 is a simplified circuit diagram representing the electricalconditions of a substrate during an exposure process in an embodiment ofthe invention;

FIG. 7 depicts a connector usable in an embodiment of the invention;

FIG. 8 is a simplified circuit diagram representing the electricalconditions of a substrate during an exposure process in an embodiment ofthe invention;

FIG. 9 is a more detailed circuit diagram representing the electricalconditions of a substrate during an exposure process in an embodiment ofthe invention;

FIG. 10 is a simplified circuit diagram representing the electricalconditions of a substrate during an exposure process in an embodiment ofthe invention;

FIG. 11 is a simplified circuit diagram representing the electricalconditions of a substrate during an exposure process in an embodiment ofthe invention; and

FIG. 12 is a simplified circuit diagram representing the electricalconditions of a substrate during an exposure process in an embodiment ofthe invention.

DETAILED DESCRIPTION

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 436, 405, 365, 248, 193, 157 or 126nm).

The term “reticle”, “mask” or “patterning device” as employed in thistext may be broadly interpreted as referring to a generic patterningdevice that can be used to endow an incoming radiation beam with apatterned cross-section, corresponding to a pattern that is to becreated in a target portion of the substrate. The term “light valve” canalso be used in this context. Besides the classic mask (transmissive orreflective, binary, phase-shifting, hybrid, etc.), examples of othersuch patterning devices include a programmable mirror array and aprogrammable LCD array.

FIG. 1 schematically depicts a lithographic apparatus. The lithographicapparatus includes an illumination system (also referred to asilluminator) IL configured to condition a radiation beam B (e.g., UVradiation or DUV radiation), a mask support (e.g., a mask table) MTconstructed to support a patterning device (e.g., a mask) MA andconnected to a first positioner PM configured to accurately position thepatterning device MA in accordance with certain parameters, a substratetable 60 constructed to hold a substrate (e.g., a resist coated wafer) Wand connected to a second positioner PW configured to accuratelyposition the substrate table 60 in accordance with certain parameters,and a projection system (e.g., a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g., comprising one ormore dies) of the substrate W.

In operation, the illumination system IL receives the radiation beam Bfrom a radiation source SO, e.g. via a beam delivery system BD. Theillumination system IL may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic,electrostatic, and/or other types of optical components, or anycombination thereof, for directing, shaping, and/or controllingradiation. The illuminator IL may be used to condition the radiationbeam B to have a desired spatial and angular intensity distribution inits cross section at a plane of the patterning device MA. The term“projection system” PS used herein should be broadly interpreted asencompassing various types of projection system, including refractive,reflective, catadioptric, anamorphic, magnetic, electromagnetic and/orelectrostatic optical systems, or any combination thereof, asappropriate for the exposure radiation being used, and/or for otherfactors such as the use of an immersion liquid or the use of a vacuum.Any use of the term “projection lens” herein may be considered assynonymous with the more general term “projection system” PS.

The lithographic apparatus is of a type wherein at least a portion ofthe substrate W may be covered by an immersion liquid having arelatively high refractive index, e.g., water, so as to fill animmersion space 10 between the projection system PS and the substrateW—which is also referred to as immersion lithography. More informationon immersion techniques is given in U.S. Pat. No. 6,952,253, which isincorporated herein in its entirety by reference.

The lithographic apparatus may be of a type having two or more substratetables 60 (also named “dual stage”). In such “multiple stage” machine,the substrate tables 60 may be used in parallel, and/or steps inpreparation of a subsequent exposure of the substrate W may be carriedout on the substrate W located on one of the substrate table 60 whileanother substrate W on the other substrate table 60 is being used forexposing a pattern on the other substrate W.

In addition to the substrate table 60, the lithographic apparatus maycomprise a measurement stage (not depicted in FIG. 1). The measurementstage is arranged to hold a sensor and/or a cleaning device. The sensormay be arranged to measure a property of the projection system PS or aproperty of the radiation beam B. The measurement stage may holdmultiple sensors. The cleaning device may be arranged to clean part ofthe lithographic apparatus, for example a part of the projection systemPS or a part of a system that provides the immersion liquid. Themeasurement stage may move beneath the projection system PS when thesubstrate table 60 is away from the projection system PS.

In operation, the radiation beam B is incident on the patterning device,e.g. mask, MA which is held on the mask support MT, and is patterned bythe pattern (design layout) present on patterning device MA. Havingtraversed the mask MA, the radiation beam B passes through theprojection system PS, which focuses the beam onto a target portion C ofthe substrate W. With the aid of the second positioner PW and a positionmeasurement system IF, the substrate table 60 can be moved accurately,e.g., so as to position different target portions C in the path of theradiation beam B at a focused and aligned position. Similarly, the firstpositioner PM and possibly another position sensor (which is notexplicitly depicted in FIG. 1) may be used to accurately position thepatterning device MA with respect to the path of the radiation beam B.Patterning device MA and substrate W may be aligned using mask alignmentmarks M1, M2 and substrate alignment marks P1, P2. Although thesubstrate alignment marks P1, P2 as illustrated occupy dedicated targetportions, they may be located in spaces between target portions.Substrate alignment marks P1, P2 are known as scribe-lane alignmentmarks when these are located between the target portions C.

In this specification, a Cartesian coordinate system is used. TheCartesian coordinate system has three axis, i.e., an x-axis, a y-axisand a z-axis. Each of the three axis is orthogonal to the other twoaxis. A rotation around the x-axis is referred to as an Rx-rotation. Arotation around the y-axis is referred to as an Ry-rotation. A rotationaround about the z-axis is referred to as an Rz-rotation. The x-axis andthe y-axis define a horizontal plane, whereas the z-axis is in avertical direction. The Cartesian coordinate system is not limiting ofthe invention and is used for clarification only. Instead, anothercoordinate system, such as a cylindrical coordinate system, may be usedto clarify the invention. The orientation of the Cartesian coordinatesystem may be different, for example, such that the z-axis has acomponent along the horizontal plane.

A controller 500 controls the overall operations of the lithographicapparatus and in particular performs an operation process describedfurther below. Controller 500 can be embodied as a suitably-programmedgeneral purpose computer comprising a central processing unit, volatileand non-volatile storage means, one or more input and output devicessuch as a keyboard and screen, one or more network connections and oneor more interfaces to the various parts of the lithographic apparatus.It will be appreciated that a one-to-one relationship betweencontrolling computer and lithographic apparatus is not necessary. Onecomputer can control multiple lithographic apparatuses. Multiplenetworked computers can be used to control one lithographic apparatus.The controller 500 may also be configured to control one or moreassociated process devices and substrate handling devices in a lithocellor cluster of which the lithographic apparatus forms a part. Thecontroller 500 can also be configured to be subordinate to a supervisorycontrol system 600 of a lithocell or cluster and/or an overall controlsystem of a fab.

Arrangements for providing liquid between a last optical element 100 ofthe projection system PS and the substrate W can be classed into threegeneral categories. These are the bath type arrangement, the so-calledlocalized immersion systems and the all-wet immersion systems.Embodiments of the present invention relate particularly, but notexclusively, to localized immersion systems.

In an arrangement which has been proposed for a localized immersionsystem, a liquid confinement structure 12 extends along at least a partof a boundary of an immersion space 10 between the last optical element100 of the projection system PS and the facing surface of the stage ortable facing the projection system PS. The facing surface of the tableis referred to as such because the table is moved during use and israrely stationary. Generally, the facing surface of the table refers toa surface of a substrate W, substrate table 60 which surrounds thesubstrate W or both.

FIGS. 2 and 3 show different features which may be present in variationsof confinement structure 12. The features described herein may beselected individually or in combination as shown or as required.

FIG. 2 shows two variants of a liquid confinement structure 12 aroundthe bottom surface of a last optical (lens) element 100; the left handside shows one variant and the right hand side another. Features of thetwo variants may be combined in a single liquid confinement structure.The last optical element 100 has an inverted frustro-conical shape 30.The frustro-conical shape 30 has a planar bottom surface and a conicalsurface. The frustro-conical shape 30 protrudes from a planar surfaceand has a bottom planar surface. The bottom planar surface is theoptically active portion of the bottom surface of the last opticalelement 100, through which the projection beam may pass. The liquidconfinement structure 12 surrounds at least part of the frustro-conicalshape 30. The liquid confinement structure 12 has an inner-surface whichfaces towards the conical surface of the frustro-conical shape 30. Theinner-surface and the conical surface have complementary shape. A topsurface of the liquid confinement structure 12 is substantially planar.The liquid confinement structure 12 may fit around the frustro-conicalshape 30 of the last optical element 100. A bottom surface of the liquidconfinement structure 12 is substantially planar and in use the bottomsurface may be parallel with the facing surface of the substrate table60 and/or the substrate W. The distance between the bottom surface andthe facing surface may be in the range of 30 to 500 micrometers,desirably in the range of 80 to 200 micrometers.

The liquid confinement structure 12 extends closer to the facing surfaceof the substrate W and the substrate table 60 than the last opticalelement 100. The immersion space 10 is therefore defined between theinner surface of the liquid confinement structure 12, the planar surfaceof the frustro-conical shape 30 and the facing surface. During use, theimmersion space 10 is filled with liquid. The liquid fills at least partof a buffer space between the complementary surfaces between the lastoptical element 100 and the liquid confinement structure 12. In anembodiment, the liquid fills at least part of the immersion space 10between the complementary inner-surface and the conical surface.

Liquid is supplied to the immersion space 10 through an opening formedin the surface of the liquid confinement structure 12. The liquid may besupplied through a supply opening 20 in the inner-surface of the liquidconfinement structure 12. Alternatively or additionally, the liquid issupplied from an under supply opening 23 formed in the undersurface ofthe liquid confinement structure 12. The under supply opening 23 maysurround the path of the projection beam and it may be formed of aseries of openings in an array. The liquid is supplied to fill theimmersion space 10 so that flow through the immersion space 10 under theprojection system PS is laminar. The supply of liquid from the undersupply opening 23 under the liquid confinement structure 12 additionallyprevents the ingress of bubbles into the immersion space 10. This supplyof liquid functions as a liquid seal.

The liquid may be recovered from a recovery opening 21 formed in theinner-surface. The recovery of the liquid through the recovery opening21 may be by application of an under pressure; the recovery through therecovery opening 21 as a consequence of the velocity of the liquid flowthrough the immersion space 10; or the recovery may be as a consequenceof both. The recovery opening 21 may be located on the opposite side ofthe supply opening 20, when viewed in plan. Additionally oralternatively, the liquid may be recovered through an overflow opening24 located on the top surface of the liquid confinement structure 12.Overflow opening 24 prevents the upper surface 22 of the immersionliquid rising too high.

Additionally or alternatively, liquid may be recovered from under theliquid confinement structure 12 through a bottom recovery opening. Thebottom recovery opening may serve to hold (or ‘pin’) a meniscus 33 tothe liquid confinement structure 12. The meniscus 33 forms between theliquid confinement structure 12 and the facing surface and it serves asborder between the liquid space and the gaseous external environment.The bottom recovery opening may be a porous member 25 or a porous platewhich may recover the liquid in a single phase flow. The bottom recoveryopening may be a series of pinning openings 32 through which the liquidis recovered. The pinning openings 32 may recover the liquid in a twophase flow.

Optionally radially outward, with respect to the inner-surface of theliquid confinement structure 12, is a gas knife opening 26. Gas may besupplied through the gas knife opening 26 at elevated speed to assistconfinement of the immersion liquid in the immersion space 10. Thesupplied gas may be humidified and it may contain carbon dioxide. Thesupplied gas may consist essentially of carbon dioxide and water vapor.Radially outward of the gas knife opening 26 is a gas recovery opening18 for recovering the gas supplied through the gas knife. Furtheropenings, for example open to atmosphere or to a gas source, may bepresent in the bottom surface of the liquid confinement structure 12.For example, further openings may be present between the gas knifeopening 26 and the gas recovery opening 18 and/or between pinningopenings 32 and the gas knife opening 26.

Features shown in FIG. 3 which are common to FIG. 2 share the samereference numbers. The liquid confinement structure 12 has an innersurface which complements the conical surface of the frustro-conicalshape 30. The undersurface of the liquid confinement structure 12 iscloser to the facing surface than the bottom planar surface of thefrustro-conical shape 30.

Liquid is supplied to the immersion space 10 through supply openings 34formed in the inner surface of the confinement structure 12. The supplyopenings 34 are located towards the bottom of the inner surface, perhapsbelow the bottom surface of the frustro-conical shape 30. The supplyopenings 34 are located inner surface, spaced. apart around the path ofthe projection beam.

Liquid is recovered from the immersion space 10 through recoveryopenings in the undersurface of the liquid confinement structure 12. Asthe facing surface moves under the liquid confinement structure 12, themeniscus 33 may migrate over the surface of the recovery opening in thesame direction as the movement of the facing surface. The recoveryopenings may be formed of a porous member 25 or a porous plate. Theliquid may be recovered in single phase. In an embodiment the liquid maybe recovered in a two phase flow. The two phase flow is received in achamber 35 within the liquid confinement structure 12 where it isseparated into liquid and gas. The liquid and gas are recovered throughseparate channels 36, 38 from the chamber 35.

An inner periphery 39 of the undersurface of liquid confinementstructure 12 extends into the immersion space 10 away from the innersurface to form a plate 40. The inner periphery 39 forms a smallaperture which may be sized to match the shape and size of theprojection beam. The plate 40 may serve to isolate liquid either side ofit. The supplied liquid flows inwards towards the aperture, through theinner aperture and then under the plate 40 radially outwardly towardsthe surrounding recovery openings.

In an embodiment the liquid confinement structure 12 may be in twoparts: an inner part 12 a and an outer part 12 b. For convenience thisarrangement is shown in the right-hand part of FIG. 3. The two parts maymove relatively to each other, in a plane parallel to the facingsurface. The inner part 12 a may have the supply openings 34 and it mayhave the overflow recovery 24. The outer part 12 b may have the plate 40and the recovery opening. The inner part 12 a may have an intermediaterecovery 42 for recovering liquid which flows between the two parts.

In a lithographic apparatus it is desirable to position the uppersurface of a substrate to be exposed in the plane of best focus of theaerial image of the pattern projected by the projection system withgreat accuracy. To achieve this, the substrate is held on a substrateholder. The surface of the substrate holder that supports the substrateis provided with a plurality of burls whose distal ends are essentiallycoplanar in a nominal support plane. The burls, though numerous, aresmall in cross-sectional area parallel to the support plane so that thetotal cross-sectional area of their distal ends is a few percent, e.g.less than 5%, of the surface area of a substrate. The burls are commonlyconical in shape but need not be. In an embodiment, the gas pressure inthe space between the substrate holder and the substrate is reducedrelative to the pressure above the substrate to create a force clampingthe substrate to the substrate holder. In an embodiment, anelectrostatic force may be setup to clamp the substrate to the substrateholder.

The burls serve several purposes. For example, if a contaminant particleis present on the substrate holder or the substrate, it is probable thatit is not located at the location of the burl and therefore does notdistort the substrate. In addition, it is easier to manufacture theburls so that their distal ends conform accurately to an essentiallyflat plane than to manufacture a large area with very low flatness.

The burls of a substrate holder wear during use. The wear is generallyuneven and therefore causes unflatness in the surface of substrates heldby a worn substrate holder. When such wear becomes excessive it isnecessary to repair or replace the substrate holder. Repair andreplacement of the substrate holder are expensive, not only due to thecost of the repair process or the manufacture of a new substrate holder,but also due to the downtime of the lithographic apparatus in order toperform the repair or replacement.

It has heretofore been thought that the major cause of wear of burls isthe process of loading and unloading substrates onto the substrateholder. However, it has been determined that oxidation of the burls ofthe substrate holder is also a significant cause of wear of burls,especially in an immersion lithographic apparatus. FIG. 4 depicts atypical pattern of wear in a substrate holder that has been used forexposures in an immersion lithographic apparatus. It will be seen thatthe pattern of wear of the substrate holder has six petal-like areasdistributed around the peripheral region of the substrate holder. Thepetal-like areas are relatively high compared to the mean height of theburls because areas outside the petal-like areas have been eroded. Toinvestigate possible causes of this pattern of wear, levels of humidityhave been simulated in the space between the substrate and the substrateholder during exposures in the immersion lithographic apparatus in whichthe substrate holder of FIG. 4 was used. The results, shown in FIG. 5,exhibit a petal-like pattern having six petal-like areas arrangedsimilarly to the pattern of wear shown in FIG. 4. The petal-like areasare areas of low humidity and are centered around the evacuation portsof the substrate holder.

Without wishing to be bound by any particular theory, it is believedthat a significant amount of wear of the burls is caused by oxidation ofthe material of the burls (SiSiC) and/or coatings thereon (e.g. ofdiamond-like carbon) in the areas of relatively high humidity. Herein,the term “oxidation of the substrate holder” is used to refer tooxidation of the burls and/or any coatings thereon that might affect theflatness of a substrate held by the substrate holder. In addition, it isbelieved that the oxidation can be accelerated by a potential differencebetween the substrate and the substrate holder, which causes a currentto flow between the substrate and substrate holder. A potentialdifference between the substrate and substrate holder can be caused bytribo-electric effects due to the flow of immersion liquid (e.g. water)across the surface of the substrate. Such a potential difference maydepend on the resist used for the photo-sensitive layer and/or anytopcoat. A potential difference between the substrate and substrateholder might also be caused by the exposure radiation (e.g. DUV) that isincident on the photo-sensitive layer.

Accordingly therefore, it is proposed to provide, in the immersionlithographic apparatus, a current control device arranged to reduce anelectric current flowing between the substrate and the substrate holderwhile the immersion liquid is confined to the immersion space 10.

In an embodiment, the current control device provides an impedance (e.g.a resistance) of greater than 10 kΩ, or greater than 100 kΩ, desirablygreater than 1 MΩ, or greater than 10 MΩ, between the substrate holderand ground. In some circumstances an impedance as high as 100 MΩ may besuitable. Increasing the impedance of the ground path can increase thetime taken for the potential of the substrate to equalize when it isloaded onto the substrate table so that the resistance of the groundpath should be no higher than necessary. In an embodiment, the impedanceof the ground path may be less than about 1 GΩ or less than about 100MΩ. FIG. 6 shows a simplified electrical circuit of such an embodiment.

The embodiment can be represented by a series circuit comprising: avoltage source 61 representing the potential difference ΔU_(surface)generated on the substrate surface by, for example, tribo-electriceffects and/or irradiation; the resistance 62 of the substrateR_(wafer); the resistance 63 between the substrate and the substrateholder R_(W2WT); and the resistance 64 between the substrate holder andground R_(ground). The circuit is completed through the ground; theupper surface of the substrate W is connected to ground via theimmersion liquid and the liquid confinement system 12.

It is believed that the oxidation of the substrate holder WT is driven,at least in part, by the potential difference ΔU_(ox) across theresistance 63 between the substrate and the substrate holder R_(W2WT).The resistance 63 between the substrate and the substrate holderR_(W2WT) and the resistance 64 between the substrate holder and groundR_(ground) form a voltage divider that divides the potential differenceΔU_(surface) generated on the substrate surface. Therefore, byincreasing the resistance R_(ground) the potential difference ΔU_(ox) isreduced, current flow between the substrate holder WT and ground isreduced and hence oxidation of the substrate holder WT is reduced.

The resistance 64 (R_(ground)) can be provided at any convenient pointin the electrical path between the substrate holder WT and ground. Forexample, the substrate holder WT is commonly electrically connected tothe substrate table 60. A multi-conductor cable provides power andsignal connections to sensors and actuators on the substrate table 60.As shown in FIG. 7, resistor 71 of suitable value can be provided in aconnector 72 between a conductor connected to the substrate table 60 anda grounded shield 73.

It will be appreciated that the lithographic apparatus is considerablymore complex than the simplified equivalent circuit depicted in FIG. 6;a more detailed representation is given in FIG. 8.

As shown in FIG. 8, substrate W is held on substrate holder WT, which inturn is mounted in a recess of substrate table 60. Substrate table 60also comprises one or more actuators 601 and sensors 602 which areelectrically connected to an electrical connector box 604. A flexiblemulti-conductor cable (often referred to as the cable slab 605) providesall necessary electrical connections, for both power and signaling,between the movable substrate table 60 and the static part of thelithographic apparatus. The cable slab 605 includes a ground connection606, into which an impedance forming the current control device of anembodiment of the present invention can be inserted. Ground connection606 is connected to the machine ground 609 via various other componentssuch as a power calibrator 607 and current monitor 608. The liquidconfinement structure 12 is grounded via the metrology frame MF, andother components such as power lockout 610, circuit breaker 611 andpower distribution cabinet 612. At certain times during exposure ofsubstrates or measurement processes, it is possible that immersionliquid is simultaneously in contact with the substrate W and sensor 602.During such times, in addition to the current 620 between substrate Wand liquid confinement structure 12, a substrate-sensor current 621 mayarise between the substrate W and the sensor 602 and a sensor-structurecurrent 622 may arise between the sensor 602 and the liquid confinementstructure 12. If the magnitudes of the substrate-sensor current 621 andthe sensor-structure current 622 are significant, they may be takenaccount of in determining a suitable value for the impedance 64 formingthe current control device.

It will be noted that cooling water used to cool various parts of thelithographic apparatus has potential to provide a parallel path toground. If the path to ground through such cooling water has aresistance lower than that of the current control device, then theeffectiveness of a passive current control device might be reduced. Thiscircumstance can be avoided by maintaining the purity of the coolingwater high, and hence its conductivity low. In the event that theconductivity of the cooling water increases sufficiently to affect theeffectiveness of the current control device it can be refreshed withpurer water. If an active current control device is used then theconductivity of the cooling water may not be important.

In an embodiment, the current control device provides an impedance (e.g.a resistance) of greater than 10 kΩ, or greater than 100 kΩ, desirably 1MΩ, or greater than 10 MΩ, between the liquid confinement structure 12and ground. FIG. 9 shows a simplified electrical circuit of such anembodiment. In this Figure, components that are equivalent to those ofFIG. 6 are indicated by the same references and not described again forthe sake of brevity. Impedance 64 is located in the electrical path fromthe liquid confinement structure 12 to ground but functions to limit thecurrent flowing between the substrate W and substrate holder WT in justthe same way as in the embodiment of FIG. 6.

In the above described embodiments, the impedance 64 forming the currentcontrol device may be a variable impedance. Such a variable impedancemay be controlled on the basis of a measurement of the current flowingto ground and/or the potential of the substrate W.

A further embodiment is shown in FIG. 10, in which parts that areequivalent to parts of earlier embodiments are given the same referencesand not described again in the interest of brevity. In this embodiment,the current control device comprises a current sensor 110, a controller111 (e.g. an operational amplifier) and a variable voltage source 112.The current sensor 110 is arranged in the electrical path between thesubstrate holder WT and ground and measures the current flowing therein.The controller 111 receives the output of the current sensor 110 andgenerates a control signal proportional thereto. The variable voltagesource 112 is arranged in the electrical path between the current sensor110 and ground, receives the control signal and sets its output voltageaccordingly. Thus the current sensor 110, controller 111 and variablevoltage source 112 are configured as a feedback loop to reduce thecurrent flowing to ground and hence the current flowing between thesubstrate W and substrate holder WT.

FIG. 11 depicts a variant of the embodiment of FIG. 10. In theembodiment of FIG. 11, the current control device, again comprisingcurrent sensor 110, controller 111 and variable voltage source 112, isarranged in the electrical path between the liquid confinement system 12and ground rather than between the substrate table 60 and ground.Nevertheless, the current control device of FIG. 11 functions in thesame way as that of FIG. 10.

In further variants (not shown in the figures), the current flowing fromsubstrate table 60 to ground can be measured and used to control avoltage applied to the liquid confinement system 12 or the current flowto ground from the liquid confinement system 12 can be measured and usedto control a voltage applied to the substrate holder WT.

In the embodiments of FIGS. 10 and 11, the variable voltage source maybe replaced by a different electrical power source, such as a currentsource. Alternatively or in addition, the current sensor 110 can bereplaced by a voltage sensor. A voltage sensor may be arranged tomeasure the potential of the substrate W or the substrate holder WT. Forexample an electrostatic voltage meter can be located in the liquidconfinement structure 12 to measure the potential of the top of thesubstrate W or in the substrate holder WT to measure the potential ofthe bottom of the substrate W.

An immersion-type lithographic apparatus may be provided with one ormore substrate table heaters in the substrate holder WT to counteractthe cooling effect of evaporation of the immersion liquid and assist inmaintaining the substrate holder WT and substrate W at a constanttemperature. Although such a substrate table heater may be electricallyinsulated from the substrate holder WT and the path to ground, theheaters may be capacitively linked to the ground path. This is depictedin FIG. 12, which is a simplified electrical equivalent circuit diagramof an embodiment including substrate table heaters, by capacitors 121and 122. Voltage source 123 represents the potential differenceΔU_(heaters) induced by the currents flowing in the substrate tableheaters, which may be AC or include a variable component.

To reduce the effect of currents flowing in the substrate table heaters,a conductive shield 124 is provided between the substrate table heatersand the ground path from the substrate holder WT and is electricallyconnected to ground. The conductive shield 124 may form a Faraday cageenclosing the substrate table heaters. The conductive shield 124 can beformed by a conductive foil or mesh wrapped around the substrate tableheaters. Alternatively or in addition, the conductive shield 124 maycomprise a conductive glue used to secure the substrate table heaters inplace.

Embodiments are provided according to the following clauses:

1. A substrate table for an immersion system comprising a projectionsystem arranged to project an image onto a substrate and a liquidconfinement system configured to confine an immersion liquid to a spacebetween the projection system and the substrate, the substrate tablecomprising:

a substrate holder configured to hold the substrate; and

a current control device arranged to reduce an electric current flowingbetween the substrate and the substrate holder whilst the immersionliquid is confined to the space.

2. The substrate table according to clause 1, wherein the currentcontrol device provides an impedance of greater than 100 kΩ, desirablygreater than 10 MΩ, between the substrate holder and ground and/orbetween the liquid confinement system and ground.

3. The substrate table according to clause 2, wherein the impedance isvariable and the current control device further comprises a controllerto control the impedance.

4. The substrate table according to clause 1, wherein the currentcontrol device comprises an electrical power source connected to atleast one of the substrate holder, the substrate table and the liquidconfinement system.

5. The substrate table according to clause 3, wherein the currentcontrol device further comprises:

a current detector to detect a current between the substrate holder andground and/or between the liquid confinement system and ground; and

a controller to control the electrical power source and/or the impedanceon the basis of the detection of current.

6. The substrate table according to clause 3, wherein the currentcontrol device further comprises:

a voltage sensor to measure a potential of a part of the substrate, thesubstrate holder and/or the liquid confinement system; and

a controller to control the electrical power source and/or the impedanceon the basis of the measured potential.

7. The substrate table according to clause 6, wherein the voltage sensoris arranged to measure a potential of the lower surface of thesubstrate; and the electrical power source is connected to the substrateholder or the substrate table.

8. The substrate table according to clause 6, wherein the voltage sensoris arranged to measure a potential of the upper surface of thesubstrate; and the electrical power source is connected to the liquidconfinement system.

9. The substrate table according to any of clauses 1-8, furthercomprising an electrical heater configured to heat at least a part ofthe substrate and wherein the current control device comprises agrounded shield arranged between the electrical heater and the substrateholder.10. The substrate table according to any of clauses 1-8, wherein thecurrent control device comprises a conductor configured to contact thesubstrate to provide a direct electrical path between the substrate andground.11. The substrate table, according to clause 10, wherein the conductorcomprises a burl having a hard conductive coating without an insulatingouter layer.12. The substrate table according to clause 11, wherein the hardconductive coating is made from at least one material selected from thegroup consisting of CrN, TiN, doped DLC, and alloys containing one ormore of Ti, W and Mo.13. The substrate table according to clause 10, wherein the conductorcomprises an actuated pin configured to lift the substrate from thesubstrate holder.

14. A lithographic apparatus comprising:

a substrate holder configured to hold a substrate;

a projection system arranged to project an image onto a substrate heldby the substrate holder;

a liquid confinement system configured to confine an immersion liquid toa space between the projection system and the substrate; and

a current control device arranged to reduce an electric current flowingbetween the substrate and the substrate holder whilst the immersionliquid is confined to the space.

15. The lithographic apparatus according to clause 14 wherein thecurrent control device provides an impedance of greater than 10 kΩ,desirably greater than 1 MΩ, between the substrate holder and groundand/or between the liquid confinement system and ground.16. The lithographic apparatus according to clause 15, wherein theimpedance is variable and the current control device further comprises acontroller to control the impedance.17. The lithographic apparatus according to clause 14 or clause 15,wherein the current control device comprises an electrical power sourceconnected to at least one of the substrate holder, the substrate tableand the liquid confinement system.18. The lithographic apparatus according to clause 16 or clause 17,wherein the current control device further comprises:

a current detector to detect a current between the substrate holder andground and/or between the liquid confinement system and ground; and

a controller to control the electrical power source and/or the impedanceon the basis of the detection of current.

19. The lithographic apparatus according to clause 16 or clause 17,wherein the current control device further comprises:

a voltage sensor to measure a potential of a part of the substrate, thesubstrate holder and/or the liquid confinement system; and

a controller to control the electrical power source and/or the impedanceon the basis of the measured potential.

20. The lithographic apparatus according to clause 19, wherein thevoltage sensor is arranged to measure a potential of the lower surfaceof the substrate; and the electrical power source is connected to thesubstrate holder or the substrate table.

21. The lithographic apparatus according to clause 19, wherein thevoltage sensor is arranged to measure a potential of the upper surfaceof the substrate; and the electrical power source is connected to theliquid confinement system.

22. The lithographic apparatus according to any of clauses 14-21,further comprising an electrical heater configured to heat at least apart of the substrate and wherein the current control device comprises agrounded shield arranged between the electrical heater and the substrateholder.23. The lithographic apparatus according to any of clauses 14-22,wherein the current control device comprises a conductor configured tocontact the substrate to provide a direct electrical path between thesubstrate and ground.24. The lithographic apparatus according to clause 23, wherein theconductor comprises a burl having a hard conductive coating without aninsulating outer layer.25. The lithographic apparatus according to clause 24, wherein the hardconductive coating is made from at least one material selected from thegroup consisting of CrN, TiN, doped DLC, and alloys containing one ormore of Ti, W and Mo.26. The lithographic apparatus according to clause 23, wherein theconductor comprises an actuated pin configured to lift the substratefrom the substrate holder.27. A device manufacturing method comprising using a lithographicapparatus having a projection system to project an image onto asubstrate held on a substrate holder, the method comprising:

using a liquid confinement to confine a liquid to a space between theprojection system and the substrate; and

reducing a current between the liquid confinement system and thesubstrate holder while the immersion liquid is confined to the space.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains one or multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that embodiments of the invention may be used inother applications.

While specific embodiments of the invention have been described above,it will be appreciated that embodiments of the invention may bepracticed otherwise than as described.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

The invention claimed is:
 1. A substrate table for an immersion systemcomprising a projection system arranged to project an image onto asubstrate and a liquid confinement system configured to confine animmersion liquid to a space between the projection system and thesubstrate, the substrate table comprising: a substrate holder configuredto hold the substrate; and a current control device arranged to reducean electric current flowing between the substrate and the substrateholder while the immersion liquid is confined to the space.
 2. Thesubstrate table according to claim 1, wherein the current control deviceprovides an impedance of greater than 100 kΩ between the substrateholder and ground.
 3. The substrate table according to claim 1, whereinthe current control device provides an impedance of greater than 1 MΩbetween the substrate holder and ground.
 4. The substrate tableaccording to claim 2, wherein the impedance is variable and the currentcontrol device further comprises a controller to control the impedance.5. The substrate table according to claim 1, wherein the current controldevice further comprises: a current detector to detect a current betweenthe substrate holder and ground; and a controller to control anelectrical power source and/or an impedance on the basis of thedetection of current.
 6. The substrate table according to claim 1,wherein the current control device further comprises: a voltage sensorto measure a potential of a part of the substrate and/or the substrateholder; and a controller to control an electrical power source and/or animpedance on the basis of the measured potential.
 7. The substrate tableaccording to claim 6, wherein the voltage sensor is arranged to measurea potential of a lower surface of the substrate, and the electricalpower source is connected to the substrate holder or the substratetable.
 8. The substrate table according to claim 6, wherein the voltagesensor is arranged to measure a potential of an upper surface of thesubstrate.
 9. The substrate table according to claim 1, wherein thecurrent control device comprises an electrical power source connected tothe substrate holder and/or the substrate table.
 10. The substrate tableaccording to claim 1, further comprising an electrical heater configuredto heat at least a part of the substrate and wherein the current controldevice comprises a grounded shield arranged between the electricalheater and the substrate holder.
 11. The substrate table according toclaim 1, wherein the current control device comprises a conductorconfigured to contact the substrate to provide a direct electrical pathbetween the substrate and ground.
 12. The substrate table according toclaim 11, wherein the conductor comprises a burl having a hardconductive coating without an insulating outer layer.
 13. The substratetable according to claim 12, wherein the hard conductive coating is madefrom at least one selected from: CrN, TiN, doped DLC, and/or an alloycontaining one or more selected from: Ti, W and/or Mo.
 14. The substratetable according to claim 11, wherein the conductor comprises an actuatedpin configured to lift the substrate from the substrate holder.
 15. Alithographic apparatus comprising: the substrate table of claim 1; aprojection system arranged to project an image onto a substrate held bythe substrate table; and a liquid confinement system configured toconfine an immersion liquid to a space between the projection system andthe substrate.
 16. A device manufacturing method comprising using alithographic apparatus having a projection system to project an imageonto a substrate held on a substrate holder, the method comprising:using a liquid confinement system to confine a liquid to a space betweenthe projection system and the substrate; and reducing an electriccurrent flowing between the substrate and the substrate holder while theimmersion liquid is confined to the space.
 17. The method according toclaim 16, wherein the reducing the electric current comprises providingan impedance of greater than 100 kΩ between the substrate holder andground and/or between the liquid confinement system and ground.
 18. Themethod according to claim 17, comprising varying the impedance using acontroller.
 19. The method according to claim 16, wherein the reducingthe electric current comprises: detecting a current between thesubstrate holder and ground; and controlling an electrical power sourceand/or an impedance on the basis of the detection of current.
 20. Themethod according to claim 16, wherein the reducing the electric currentcomprises: measuring a potential of a part of the substrate and/or thesubstrate holder; and controlling an electrical power source and/or animpedance on the basis of the measured potential.